Nitrogen-rich graphitic-carbon@graphene as a metal-free electrocatalyst for oxygen reduction reaction

In situ N-doping engineered biochar catalysts for oxidation degradation of sulfadiazine via nonradical pathways: Singlet oxygen and electron transfer

Understanding the structure of non-metallic heteroatom-doped carbon catalysts and the subsequent degradation of new pollutants is crucial for designing more efficient carbon catalysts. Environmentally friendly in situ N-doped biochar catalysts were prepared for peroxymonosulfate (PMS) activation and sulfadiazine (SDZ) degradation. The acid washing process and calcination temperature of catalyst increased π-π* shake up, graphitic N percentage, specific surface area and defects, promoting the transformation of pollutant degradation mechanism from radical pathway to non-radical pathway. 100 % of the SDZ with the initial concentration of 10 mg/L was quickly degraded within 60 min using 0.2 g/L catalysts and 0.5 mM PMS. Excellent catalytic performance was attributed to singlet oxygen and electron transfer-dominated non-radical pathways. The four potential degradation pathways of SDZ were proposed, and toxicity predication indicated that overall biotoxicity of the intermediates during SDZ degradation was decreased. This research deepens our understanding of the mechanisms of non-radical pathways and guides the synthesis of carbon-based catalysts.

Revision of the oxygen reduction reaction on N-doped graphenes by grand-canonical DFT

Nitrogen-doped graphenes were among the first promising metal-free carbon-based catalysts for the oxygen reduction reaction (ORR). However, data on the most efficient catalytic centers and their catalytic mechanisms are still under debate. In this work, we study the associative mechanism of the ORR in an alkaline medium on graphene containing various types of nitrogen doping. The free energy profile of the reaction is constructed using grand-canonical DFT at a constant electrode potential in combination with an implicit electrolyte model. It is shown that the reaction mechanism differs from the generally accepted one and depends on the surface potential and doping type. In particular, as the potential decreases, coupled electron-proton transfer changes to sequential electron and proton transfer, and the potential at which this occurs depends on the doping type. It has been shown that oxygen chemisorption is the limiting step. The electrocatalytic mechanism of the nitrogen dopants involves reducing the oxygen chemisorption energy. Calculations predict that, at different potentials, different types of nitrogen impurities most effectively catalyze the ORR.

Bandgap Engineering of Melon using Highly Reduced Graphene Oxide for Enhanced Photoelectrochemical Hydrogen Evolution

The uncondensed form of polymeric carbon nitrides (PCN), generally known as melon, is a stacked 2D structure of poly(aminoimino)heptazine. Melon is used as a photocatalyst in solar energy conversion applications, but suffers from poor photoconversion efficiency due to weak optical absorption in the visible spectrum, high activation energy, and inefficient separation of photoexcited charge carriers. Experimental and theoretical studies are reported to engineer the bandgap of melon with highly reduced graphene oxide (HRG). Three HRG@melon nanocomposites with different HRG:melon ratios (0.5%, 1%, and 2%) are prepared. The 1% HRG@melon nanocomposite shows higher photocurrent density (71 µA cm-2 ) than melon (24 µA cm-2 ) in alkaline conditions. The addition of a hole scavenger further increases the photocurrent density to 630 µA cm-2 relative to the reversible hydrogen electrode (RHE). These experimental results are validated by calculations using density functional theory (DFT), which revealed that HRG results in a significant charge redistribution and an improved photocatalytic hydrogen evolution reaction (HER).

Pyrolysis Enzymolysis-Treated Pomelo Peel: Porous Carbon Materials with Fe-Nx Sites for High-Performance Supercapacitor and Efficient Oxygen Reduction Applications

This paper proposes a different strategy for deriving carbon materials from biomass, abandoning traditional strong corrosive activators and using a top-down approach with a mild green enzyme targeted to degrade the pectin matrix in the inner layer of pomelo peel cotton wool, inducing a large number of nanopores on its surface. Meanwhile, the additional hydrophilic groups produced via an enzymatic treatment can be used to effectively anchor the metallic iron atoms and prepare porous carbon with uniformly dispersed Fe-Nx structures, in this case optimizing sample PPE-FeNPC-900's specific surface area by up to 1435 m2 g-1. PPE-FeNPC-900 is used as the electrode material in a 6 M KOH electrolyte; it manifests a decent specific capacitance of 400 F g-1. The assembled symmetrical supercapacitor exhibits a high energy density of 12.8 Wh kg-1 at a 300 W kg-1 power density and excellent cycle stability. As a catalyst, it also exhibits a half-wave potential of 0.850 V (vs. RHE) and a diffusion-limited current of 5.79 mA cm-2 at 0.3 V (vs. RHE). It has a higher electron transfer number and a lower hydrogen peroxide yield compared to commercial Pt/C catalysts. The green, simple, and efficient strategy designed in this study converts abundant, low-cost waste biomass into high-value multifunctional carbon materials, which are critical for achieving multifunctional applications.

Electrocatalytic Oxygen Reduction Reaction of Graphene Oxide and Metal-Free Graphene in an Alkaline Medium

Graphene is a well-known two-dimensional material with a large surface area and is used for numerous applications in a variety of fields. Metal-free carbon materials such as graphene-based materials are widely used as an electrocatalyst for oxygen reduction reactions (ORRs). Recently, more attention has been paid to developing metal-free graphenes doped with heteroatoms such as N, S, and P as efficient electrocatalysts for ORR. In contrast, we found our prepared graphene from graphene oxide (GO) by the pyrolysis method under a nitrogen atmosphere at 900 °C has shown better ORR activity in aqueous 0.1 M potassium hydroxide solution electrolyte as compared with the electrocatalytic activity of pristine GO. At first, we prepared various graphene by pyrolysis of 50 mg and 100 mg of GO in one to three alumina boats and pyrolyzed the samples under a N₂ atmosphere at 900 °C. The prepared samples are named G50-1B to 3B and G100-1B and G100-2B. The prepared GO and graphenes were also analyzed under various characterization techniques to confirm their morphology and structural integrity. The obtained results suggest that the ORR electrocatalytic activity of graphene may differ based on the pyrolysis conditions. We found that G100-1B (E_onset, E_1/2, J_L, and n values of 0.843, 0.774, 4.558, and 3.76) and G100-2B (E_onset, E_1/2, and J_L values of 0.837, 0.737, 4.544, and 3.41) displayed better electrocatalytic ORR activity, as did Pt/C electrode (E_onset, E_1/2, and J_L values of 0.965, 0.864, 5.222, and 3.71, respectively). These results display the wide use of the prepared graphene for ORR and also can be used for fuel cell and metal-air battery applications.

One-Step Synthesis of Aminobenzoic Acid Functionalized Graphene Oxide by Electrochemical Exfoliation of Graphite for Oxygen Reduction to Hydrogen Peroxide and Supercapacitors

Graphene-based materials have attracted considerable attention as promising electrocatalysts for the oxygen reduction reaction (ORR) and as electrode materials for supercapacitors. In this work, electrochemical exfoliation of graphite in the presence of 4-aminebenzoic acid (4-ABA) is used as a one-step method to prepare graphene oxide materials (EGO) functionalized with aminobenzoic acid (EGO-ABA). The EGO and EGO-ABAs materials were characterized by FT-IR spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, X-ray diffraction and scanning electron microscopy. It was found that the EGO-ABA materials have smaller flake size and higher density of oxygenated functional groups compared to bare EGO. The electrochemical studies showed that the EGO-ABA catalysts have higher activity for the ORR to H2O2 in alkaline medium compared to EGO due to their higher density of oxygenated functional groups. However, bare EGO has a higher selectivity for the 2-electron process (81%) compared to the EGO-ABA (between 64 and 72%) which was related to a lower content of carbonyl groups. The specific capacitance of the EGO-ABA materials was higher than that of EGO, with an increase by a factor of 3 for the materials prepared from exfoliation in 5 mM 4-ABA/0.1 M H2SO4. This electrode material also showed a remarkable cycling capability with a loss of only 19.4% after 5000 cycles at 50 mVs−1.

Radical and non-radical cooperative degradation in metal-free electro-Fenton based on nitrogen self-doped biochar

To achieve sustainable metal-free electron-Fenton, N self-doped biochar air-cathode (BCAC) was prepared by pyrolyzing coffee residues. During the pyrolysis process, the endogenous N transformed from edge-doping to graphite-doping. Particularly, N vacancies started to evolve when the peak temperature exceeded 700 °C. A high Tetracycline removal rate of 70.42% was obtained on the BCAC at the current density of 4 mA cm^-2. Quenching tests incorporated with ESR spectroscopy were adopted to identify the specific oxidants produced on the cathode. The results showed that •OH (37.36%), •O₂^- (29.67%) and ¹O₂ (24.17%) played comparable role in the tetracycline removal, suggesting the coexist of radical and non-radical oxidants in our electro-Fenton system. According to the structure characterization and the DFT calculation, graphitic N was suggested as the critical site for H₂O₂ generation, and both graphitic N and pyridinic N were electroactive sites for H₂O₂ activation to •OH. Graphitic N and N vacancies with stronger capabilities in O₂ adsorption and electron-trapping were proposed as the electroactive sites for ¹O₂ and •O₂^- formation. This work predicts a novel electro-Fenton process with cooperative radical and non-radical degradation on N self-doped carbonaceous catalysts at a mild condition, which is extremely meaningful for boosting sustainable electro-Fenton technology.

Unveiling the Redox Electrochemistry of MOF-Derived fcc-NiCo@GC Polyhedron as an Advanced Electrode Material for Boosting Specific Energy of the Supercapattery

Metal organic frameworks (MOFs), which constitute a new class of porous organic-inorganic hybrid materials, have gained considerable attention in the fields of electrochemical energy storage and conversion devices owing to their open topological structures, large surface areas, tunable morphologies, and extreme redox activity. A synthesis protocol that comprises coprecipitation followed by controlled calcination processes to design a battery-type electrode is used. This electrode consists of three-dimensional (3D), ant cave-like polyhedrons of nickel-cobalt alloy on graphitic carbon (GC; NiCo@GC) nanostructures; trimesic acid is used as a potential MOF-linker. The developed NiCo@GC sample exhibits mesoporous characteristics with the maximum surface area of 94.08 m² g^-1 at 77 K. In addition, the redox activity at different sweep rates reveals the battery-type charge storage behavior of the NiCo@GC electrode; its three-electrode assembly provides 444 C g^-1 specific capacity at 2 A g^-1 with long-term capacity retention. The constructed supercapattery (SC) devices (i.e., AC//NiCo@GC) achieved capacity, specific energy, and specific power are 74.3 mAh g^-1 , 39.5 Wh kg^-1 , and 665 W kg^-1 , respectively. Owing to its reasonable electrochemical characteristics, the prepared NiCo@GC material is a promising candidate for supercapattery electrodes for portable electronic devices.

Highly active oxygen reduction reaction on Fe-nanoclustered hierarchical porous carbon derived from CO2

This work presents a highly active electrocatalyst of CO₂-derived hierarchical carbon. It retains a high surface area and enables homogeneous insertion of Fe-N-C active sites for the oxygen reduction reaction. The electrocatalyst with a highly interconnected porous structure provides oxygen reduction reaction activity with an E_1/2 difference of 10 mV and a high current density equivalent to Pt/C 20 wt%.

Multiscale Understanding of Covalently Fixed Sulfur-Polyacrylonitrile Composite as Advanced Cathode for Metal-Sulfur Batteries

Metal-sulfur batteries (MSBs) provide high specific capacity due to the reversible redox mechanism based on conversion reaction that makes this battery a more promising candidate for next-generation energy storage systems. Recently, along with elemental sulfur (S8 ), sulfurized polyacrylonitrile (SPAN), in which active sulfur moieties are covalently bounded to carbon backbone, has received significant attention as an electrode material. Importantly, SPAN can serve as a universal cathode with minimized metal-polysulfide dissolution because sulfur is immobilized through covalent bonding at the carbon backbone. Considering these unique structural features, SPAN represents a new approach beyond elemental S8 for MSBs. However, the development of SPAN electrodes is in its infancy stage compared to conventional S8 cathodes because several issues such as chemical structure, attached sulfur chain lengths, and over-capacity in the first cycle remain unresolved. In addition, physical, chemical, or specific treatments are required for tuning intrinsic properties such as sulfur loading, porosity, and conductivity, which have a pivotal role in improving battery performance. This review discusses the fundamental and technological discussions on SPAN synthesis, physicochemical properties, and electrochemical performance in MSBs. Further, the essential guidance will provide research directions on SPAN electrodes for potential and industrial applications of MSBs.

Graphitic nitrogen in carbon catalysts is important for the reduction of nitrite as revealed by naturally abundant 15N NMR spectroscopy

Metal-free nitrogen-doped carbon is considered as a green functional material, but the structural determination of the atomic positions of nitrogen remains challenging. We recently demonstrated that directly-excited solid state 15N NMR (ssNMR) spectroscopy is a powerful tool for the determination of such positions in N-doped carbon at natural 15N isotope abundance. Here we report a green chemistry approach for the synthesis of N-doped carbon using cellulose as a precursor, and a study of the catalytic properties and atomic structures of the related catalyst. N-doped carbon (NH3) was obtained by the oxidation of cellulose with HNO3 followed by ammonolysis at 800 °C. It had a N content of 6.5 wt% and a surface area of 557 m2 g-1, and 15N ssNMR spectroscopy provided evidence for graphitic nitrogen besides regular pyrrolic and pyridinic nitrogen. This structural determination allowed probing the role of graphitic nitrogen in electrocatalytic reactions, such as the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and nitrite reduction reaction. The N-doped carbon catalyst (NH3) showed higher electrocatalytic activities in the OER and HER under alkaline conditions and higher activity for nitrite reduction, as compared with a catalyst prepared by the carbonization of HNO3-treated cellulose in N2. The electrocatalytic selectivity for nitrite reduction of the N-doped carbon catalyst (NH3) was directly related to the graphitic nitrogen functions. Complementary structural analyses by means of 13C and 1H ssNMR, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and low-temperature N2 adsorption were performed and provided support to the findings. The results show that directly-excited 15N ssNMR spectroscopy at natural 15N abundance is generally capable of providing information on N-doped carbon materials if relaxation properties are favorable. It is expected that this approach can be applied to a wide range of solids with an intermediate concentration of N atoms.

Carbon-nanotube-entangled Co,N-codoped carbon nanocomposite for oxygen reduction reaction

The design of highly efficient and stable electrocatalysts for oxygen reduction reaction (ORR) is still a great challenge. Herein, we prepared Co,N-codoped carbon nanocomposites (Co@NC-ZM) with entangled carbon nanotubes. The large Brunauer-Emmett-Teller surface area (604.7 m² g^-1), rich mesoporous feature, Co,N doping and synergetic effect between various species of Co@NC-ZM can expose more active sites and facilitate conductivity and mass transport. Benefiting from the above unique advantages, Co@NC-ZM exhibits excellent ORR performance with more positive onset potential (0.96 V) and half-wave potential (0.83 V) than those of commercial Pt/C (0.96 and 0.81 V, correspondingly). This work provides a new strategy for further exploring efficient non-precious-metal-based catalysts for ORR.

Toward pH Independent Oxygen Reduction Reaction by Polydopamine Derived 3D Interconnected, Iron Carbide Embedded Graphitic Carbon

Recent advancements on the development of nonprecious electrocatalysts with iron (Fe) incorporated active centers have generated confidence on realizing cost-effective proton exchange membrane fuel cells (PEMFCs). However, most of these catalysts that emerged as a substitution for the platinum supported on carbon (Pt/C) catalysts in oxygen reduction reaction (ORR) are active under basic conditions, and their feasibility in PEMFCs remains as a challenge. In this scenario, this work reports the synthesis of a Pt-free oxygen reduction electrocatalyst prepared by the annealing of polydopamine grown melamine foam. The prepared catalyst has a three-dimensional (3D) interconnected bilayer network structure possessing the carbon nitride backbone wrapped by graphitic carbon layer bearing iron carbides and nitrides as the active centers (3D-FePDC). Interestingly, the 3D-FePDC catalyst displayed an ORR activity both under acidic and basic conditions. Whereas the ORR performance of 3D-FePDC closely matches that of the commercial Pt/C in the basic medium, it displays only a low overpotential value of 60 mV under acidic conditions compared to its Pt counterpart. The kinetics of ORR on 3D-FePDC is found to be similar to the four-electron (4e) reduction pathway displayed by Pt/C. Testing of a PEMFC in a single cell mode by using 3D-FePDC as the cathode catalyst and Nafion membrane delivered a maximum power density of 278 mW cm^-2, which is a promising value expected from a system based on the nonprecious metal cathode. Ultimately, as a cost-effective catalyst that can effectively perform irrespective of the pH conditions, 3D-FePDC offers significant prospects in the areas like fuel cells and metal-air batteries which work in acidic and/or basic conditions.

Amino-1H-tetrazole-regulated high-density nitrogen-doped hollow carbon nanospheres for long-life Zn-air batteries

High-density nitrogen-doped porous carbon catalysts have been regarded as promising alternatives to precious metals in proton-exchange membrane fuel cells (PEMFC) and metal-air batteries based on the oxygen reduction reaction (ORR). We herein synthesized high-density pyridinic and graphitic N-doped hollow carbon nanospheres (G&P N-HCS) using a high-yield amino-1H-tetrazole (ATTZ) via a self-sacrificial-template method. The synthesized G&P N-HCS shows a high N content (15.2 at%), in which pyridinic (Pr) and graphitic (Gr) N are highly reactive for the ORR catalysis. We found that the half-wave potential and limiting current density of G&P N-HCS are comparable to the state-of-the-art Pt/C, whereas its cyclic durability is much superior to that of Pt/C. Experimental results indicate that an optimal ratio (1 : 1) between Gr N and Pr N in G&P N-HCS exhibits the highest ORR performances, rather than Gr N-dominated N-HCS or Pr N-dominated N-HCS. Notably, N-HCS containing only Gr N and Pr N has poor catalytic performance for ORR in alkaline electrolytes. Density functional theory (DFT) simulations untangle the catalytic nature of Pr and Gr N and decipher the relations between the N type(s) and total N content required for the ORR catalysis. This study provides a new way to design efficient N-doped porous carbon-enriched active sites, and solves the cathode catalyst in the commercialization of PEMFC and metal-air batteries.